Just like people, lawns need a balanced diet, too. If you feed them too much, too little, or apply the wrong kind of fertilizer, they won't be healthy. When you fertilize is critical, too. (Fall is better than spring.)

Test your soil. A soil test will tell you how much (if any) phosphorus (P) and potassium (K) fertilizer your lawn needs. Contact your local extension or the Cornell Nutrient Analysis Laboratory for more information.

If tests indicate that no P or K is needed, use nitrogen fertilizer sources that contain little or no P and K.

Adjust pH, if needed. Lawns should have a slightly acid pH, between 6.0 and 7.0. If your soil tests fall outside of this range, follow instructions for adding lime or sulfur to bring pH into this range.

Focus on fall. If phosphorus and potassium levels are adequate in the soil, nitrogen (N) is the most important nutrient for grass growth. Understanding how grass grows is important when making decisions about how much and when to apply nitrogen fertilizer.

For most low-maintenance lawns, a single application (1 lb. N/1,000 square feet) between Halloween and Thanksgiving (about 2 weeks after your last mowing) is the best time. Most home lawns with modest expectations do just fine with a single late-fall fertilization.

Do not fertilize during "Indian summer" - a warm period following hard frost. This may cause excessive topgrowth, reduce root storage and increase winterkill. The best window for fall application is after topgrowth stops, usually after 10 days with average daily temperatures below 50° F. Roots will continue growing and taking up fertilizer until the ground freezes.

Organic nitrogen sources are not a good choice for fall fertilization because they require warm soil and microbial action to release nitrogen. Soluble nitrogen sources are readily available to the plant, but on sandy soils there is the risk of leaching. A 50%-50% or 70%-30% mix of slow-release to quick-release N is less risky to the environment.

For higher maintenance lawns, you can also apply 1 lb. N/1,000 sq. ft. around Labor Day and/or Memorial Day. Avoid early-spring applications. Research shows that these applications do not really enhance spring green-up compared with late-fall applications. (Neglected lawns or sods thinned by winterkill may benefit from .5 lb. N/1,000 sq. ft. after the soil has thawed and drained but before the grass greens up.) At least 50 to 75 percent of the nitrogen applied to any lawn should come between the months of August and November.

Fertilizing healthy lawns in spring just increases topgrowth (and mowing chores) at the expense of root growth. The lush, succulent growth encouraged by spring fertilization makes the plant more susceptible to insects and diseases. Plants with smaller roots are also more vulnerable to drought later in the season.

Lawns that did not receive fall fertilizer applications or have suffered from winter injury may benefit from spring nitrogen applications. But wait until soil temperatures have warmed to at least 55° F before applying.

Water it in. Give your lawn a quarter- to a half-inch of water after spreading fertilizer to get the material into the ground where it can be used by plants.

Consider the source. Most synthetic lawn fertilizers contain at least 40% slow-release nitrogen. Slow-release N becomes available to the plant over a period of time depending on soil moisture, temperature, and microbial activity. The balance of the N is water soluble nitrogen, which is readily available for plant uptake.

In addition to supplying N over a longer period of time, slow-release nitrogen sources have a lower risk of burning plants and a lower potential to pollute water than water-soluble N sources. The tradeoff is that slow-release N is usually more expensive.

Natural organic fertilizers supply nitrogen in complex organic forms that are not immediately available to plants. They require warm, moist soils for microbial activity to release N. Natural organic fertilizers are well-suited for applications during warm summer months when the potential for burning plants with high-salt synthetic fertilizers is higher.

Lawns grown on mostly sandy soils should rely more on slow-release nitrogen to reduce the possibility of N leaching out of the root zone. Research shows that on most soils with some silt and clay, nitrogen leaching from lawns is rare.

Consider different needs. High-traffic areas usually require more fertilizer than low-traffic areas. Different species of grass have different needs, too. Kentucky bluegrass, for example, requires more nitrogen than fine leaf fescues.

If bluegrass doesn't get enough N, it is less competitive against weeds and pests. If fine leaf fescues (which normally grow slowly) get too much N, they produce lush, weak growth that is susceptible to pests.

Apply with care. The whole idea is to get the right amount on the lawn and none in our streams and lakes. Rotary spreaders cover a wide swath, but they can also hurl fertilizer into streets and driveways where the next rain carries it into storm drains and then on to waterways. A drop spreader may take a little longer, but it puts the fertilizer exactly where you want it. Use care when loading spreaders. Sweep up spills before they become a pollution problem.